This invention relates to devices for optical communication systems and, in particular, to an improved electro-optic device including a buffer layer of transparent conductive material.
Electro-optic devices are critical components of optical communication systems. By electrically changing the refractive index of material in an optical pathway, they can switch, attenuate or modulate an optical signal.
A commonly used electro-optic device uses an electrical field to control a waveguide path at the surface of an electro-optic crystal such as lithium niobate or lithium tantalate. The waveguide path is formed by locally doping the crystal to increase the refractive index. The electrical field applied to the waveguide region can further vary the refractive index in the path. The electrode for applying the field is typically separated from the crystal by a dielectric buffer layer to prevent absorption of guided light by the electrode metal.
FIG. 1 illustrates a conventional electro-optic modulator comprising an electro-optic crystal substrate 1 including, at the surface, an optical waveguide path 2 having a greater refractive index than the surrounding crystal. A signal electrode 4 and a ground electrode 5 are provided for controlling the electrical field in the region of waveguide path 2, and an optically transparent dielectric blanket layer 3 having a refractive index smaller than the waveguide is disposed between the waveguide 2 and the electrodes to prevent absorption of guided light by the electrode metal. A traveling wave electrode and signal source 6 are connected by a coaxial cable 7. Similarly a terminal resister 8 is connected by the coaxial cable 7. Crystal blocks 9 are bonded to the end surfaces of the waveguide path 2, and the path is connected to optical fiber segments 11 by fiber-fixing jigs 10.
FIG. 2 shows a cross section of the FIG. 1 device along the line A-xc3x81. In typical modulators, the electro-optic crystal substrate 1 is lithium niobate (LiNbO3) cut so that an X axis of the crystal extends in a longitudinal direction, and a Z axis extends in the direction of thickness. Alternatively, the electro-optic material can be LiTaO3, BaTiO3, PbTiO3, K2Li2Nb5O15 or CaNb2O7. The waveguide path 2 is formed in the crystal by doping with titanium (Ti) and configured in two arms as a Mach-Zehnder interferometer. The electrodes are gold (Au) and the buffer layer is SiO2. This and similar modulators and their fabrication are described in greater detail in U.S. Pat. No. 5,680,497 issued to M. Seino et al. on Oct. 12, 1997, which is incorporated herein by reference.
A difficulty with conventional electro-optic devices such as the above described modulators is that charge can build up at the interface between the dielectric layer and the crystal substrate. Lithium niobate, for example, is pyroelectric. Upon temperature cycling, this pyroelectricity can lead to a charge buildup at the interface which shifts the electric field required to produce a particular optical response. Such charge buildup deteriorates device performance.
Efforts have been made to reduce charge build up by doping the SiO2 buffer oxide layer to provide weak charge transport via deep traps or ion migration. A problem with a doped SiO2 buffer layer is that the conductivity of the layer varies with time due to either the filling and emptying of traps or to the motion of ions. This time dependence leads to changes in device performance over time and temperature. A second problem relates to the diffusion of Li ions into the SiO2 buffer layer from the substrate. These ions can move in an electric field and if free carriers do not screen the ions, the ions can affect the electric field seen at the waveguide and degrade device performance. Accordingly, there is a need for improved electro-optic devices having enhanced operating stability
In accordance with the invention, an electro-optic device comprising an electro-optic crystal substrate, an optical waveguide path in the crystal adjacent the substrate surface and an electrode spaced from the surface by a buffer layer is provided with enhanced operating stability by forming the buffer layer of a transparent electronically conductive material. Preferred buffer materials are electronically conductive gallium-indium-oxide and electronically conductive zinc-induim-tin-oxide.